This invention relates to atomic and molecular resonator apparatus, and more particularly, to a system which quickly, accurately and automatically finds and identifies the desired resonant peak in the response of a resonator apparatus.
Atomic and molecular resonators are basic frequency determining elements in stable frequency standards. They are widely used and both rely upon the predicted frequency of state transitions to serve as a standard.
Although the following discussion describes the operation of a cesium atomic beam tube, the present invention is applicable to other atomic and molecular resonator systems employing Ramsey-type interrogation techniques, as will be shown.
Fundamentally, an atomic beam frequency standard detects resonance transitions between specific energy states of the atom to obtain a standard frequency, while molecular energy states are employed in molecular resonators. To utilize this resonance, atomic particles, such as cesium atoms, in a beam interact with electromagnetic radiation in such a manner that when the frequency of the applied electromagnetic radiation is at the resonance frequency associated with a change of state in the particular atoms, the atoms in selected atomic states are deflected into a suitable detector. The frequency of the applied radiation is modulated about the precise atomic resonance frequency to produce a signal from the detector circuitry suitable for the servo control of a flywheel oscillator. Control circuitry is thus employed to lock the center frequency of the applied radiation to the atomic resonance line. In molecular resonators, such control circuitry locks onto the molecular resonance line.
When cesium atoms are employed in an atomic beam tube, the particular resonance of interest is that of the transition between two hyperfine levels resulting from the interaction between the nuclear magnetic dipole and the spin magnetic dipole of the valence electron. Only two stable configurations of the cesium atom exist in nature, in which the dipoles are either parallel or anti-parallel, corresponding to two allowed quantum states. Thus, in the absence of an external magnetic field, there are two hyperfine energy levels, each of which may be split by an external magnetic field into a number of Zeeman sublevels.
To cause a transition from one state to the other, an amount of energy E equal to the difference in energy of orientation must be either given to or taken from the atom. Since all cesium atoms are identical, E is the same for every atom. The frequency f of the electromagnetic energy required to cause a change of state is given by the equation E=hf, where h is Planck's constant. For cesium, the magnitude of f is approximately 9,192.631770 megahertz.
A conventional cesium atomic beam apparatus provides a source from which cesium evaporates through a collimator which forms the vapor into a narrow beam and directs it through the beam tube.
This collimated beam of atoms is acted upon by a first state selecting magnet or "A" magnet, which provides a strongly inhomogeneous magnetic field. The direction of the force experienced by a cesium atom in such a field depends on the state of the atom. In this field, the energy states F=3 and F=4 are split up into sublevels. All of the atoms of the F=4 state, except those for which m.sub.F =-4, are deflected in one direction, and all other atoms are deflected in the other direction.
Upon emergence from the A-field, those atoms enter a central region where they are subjected to a weak uniform C-field to assure the separation in energy of the m.sub.F =0 states from the nearby states for which m.sub.F .noteq.0. This small magnetic field also serves to establish the spatial orientation of the selected cesium atoms and, therefore, the required direction of the microwave magnetic field.
While in this uniform weak field region, the cesium beam is subjected to an oscillating externally generated field of approximately the resonance frequency required to cause transitions from the (3,0) to the (4,0) sublevel.
After leaving this energy transfer region, the beam is acted on by a second state-selecting magnet, similar to the A-magnet, producing a strong inhomogeneous field. Here the atoms of all the F=3 groups (and also those of the (4,-4) sublevel) are discarded. The only undiscarded atoms are those of the (4,0) sublevel, which exist at this point only because of the induced transition described above. These atoms are allowed to proceed toward a detector of any suitable type, preferably of the hot-wire ionizer mass spectrometer type.
The magnitude of the detector current, which is critically dependent upon the closeness to resonance of the applied RF frequency, is used after suitable amplification to drive a servo system to control the frequency of the oscillator/multiplier which produces the applied RF frequency.
Although the above description has been provided for an atomic beam resonator apparatus using cesium, similar systems are found in molecular beam resonator apparatus, as is known to those skilled in the art. The Ramsey type interrogation technique may also be realized in atomic and molecular resonators by application of a time delayed coherent pulse technique, rather than utilizing a beam technique.
One critical factor in both atomic and molecular resonators is the provision of the externally generated field having a frequency equal to that of the resonance frequency. In determining the resonance frequency a servo system is employed which locks onto a peak output of the beam tube, it being assumed that the peak output is approximately the resonance frequency of cesium.
Although, this specification makes reference to peaks and valleys, it should be understood that peaks are referring to maximum positive amplitudes and valleys to maximum negative amplitudes. For both atomic and molecular resonators, the response curve of FIG. 2 shows the response about a given center frequency. The response curve can be normalized about a zero reference level, so that there will be positive and negative "peaks". It is in this sense that the term peak is meant to denote a maximum, positive or negative amplitude.
It has been determined that the resonator produces a harmonic like response in which there are several peaks each spaced from the other, while there is only one maximum peak at the true resonant frequency of 9.19263177 GHz for cesium. The frequency will be different for each type molecular or atomic resonator employed. Since the frequency ranges are of such high order, it has been difficult to accurately locate the true peak during critical times. Such times occur when the standard is first started, such as when first being turned on, or when being restarted after an interruption of operation. Also, there are times when the operator must work with the resonator which also requires restarting of the apparatus. Still additionally, resonator components are replaced which causes shifts in the resonator. Even further resonators employed in frequency standards must be replaced, and readjustment of the instrument electronics must take place.
Generally, the feedback system employed to stabilize the interrogating frequency signal has locked on to one of the peaks, and the operator may attempt to finely tune the apparatus to have the feedback circuit lock on the highest peak. This work is tedious, time consuming, unreliable and generally most custom tuning is inaccurate since there has been no prior art system for ensuring that the locked-on peak is that of maximum amplitude.
Additionally, it is necessary to perform the tuning and selection more than once from the resonator's initial testing and validation to its final assembly into a complete instrument. Instrumentation for the selection process is quite complicated, difficult to use and often unreliable.
An object of this invention is to provide a system to identify the maximum resonant peak in the response of an atomic and molecular resonator.
Another object of this invention is to provide such a system which is quick, efficient, reliable, automatic and capable of being easily used.
Still another object of this invention is to provide such a system which is capable of being produced as a small modular assembly capable of being effectively operated and attached to a respective resonator so as to always be used for the same resonator during the selection process. Additionally, such a modular assembly may be used with comparable replacement resonators without adjustment of system parameters.
Other objects, advantages and features of this invention will become more apparent from the following description.